The present invention relates to a multi-layer printed circuit board. More particularly, the invention relates to a high density and remoldable multi-layer printed circuit board which is capable of mounting a mulit-lead element such as a Large Scale Integrated Circuit or LSI.
Present multi-layer printed circuit boards are formed with very high density, so that the number of boards used is reduced, resulting in an increased cost per board. Thus, when a minor change is required in the printed circuit, it is desirable to make a minor modification in the necessary portion of the fabricated board, instead of entirely remolding a new board. A method has been devised to attain this purpose.
In the devised method, the printed patterns for modification are provided at the surface layer and the pattern to be cut is cut with a knife at the surface of board. The modification is then undertaken by providing wire bonding between the patterns to be connected. However, since it is necessary, after mounting the LSI, to also provide modification patterns in portions hidden under the LSI, modification of board is impossible once the LSI is mounted. This is due to the fact that although it is desirable to provide the modification patterns in correspondence with the lead pads for bonding the leads of the LSI, if all the modification patterns are provided outside the LSI mounting portion, the plated through holes for guiding modification patterns to the internal layers of the board are concentrated in surrounding the mounting portion of the LSI. This makes it inconvenient to design wiring patterns of the internal layers. It is thus necessary to provide the modification patterns inside or outside the LSI mounting portion with adequate distribution.
The principal object of the invention is to provide a multi-layer printed circuit board which permits alteration of wiring patterns with facility, convenience and ease, even after a multi-lead element such as a LSI is mounted.
An object of the invention is to provide a multi-layer printed circuit board ensuring excellent wiring efficiency, wherein all modification patterns are provided at the outside of the LSI mounting portion and wiring patterns of internal layers may be designed with facility, convenience and ease.
Another object of the invention is to provide a multi-layer printed circuit having through holes for guiding modification patterns to the internal layer which are not concentrated in surrounding the LSI mounting portion, despite the fact that all the modification patterns are provided outside the LSI mounting portion.